1. Field of the Invention
The present invention relates to a pattern observation method, more specifically to, a pattern observation method for observing semiconductor device patterns by irradiating them with charged particle beams.
2. Background Art
In recent years, with a progress in fine patterning and multi-layer processing of semiconductor devices, a pattern of holes (contact holes, via holes, etc.) having a large aspect ratio have been used in many cases. To accommodate this trend, it is increasingly necessary to observe the processing state of a pattern bottom surface, for example, to confirm that an insulator on the bottom surface of a hole pattern is sufficiently removed or check if a width of the hole pattern bottom surface satisfies specifications.
As such a fine structure observation method in the semiconductor process, an optical method and a method using a charged-particle beam are available.
First, as one of the optical pattern observation methods, a method referred to as scatterometry is known (see, for example, Japanese Patent Application Laid-Open No. 2002-260994). By this method, a pattern to be observed is irradiated with light and then its shape is guessed from a spectrum of the reflected light. Since this method utilizes the diffraction of light, a target to be observed needs to be a regular pattern. Therefore, this method cannot observe irregular patterns such as those of system LSI device patterns. Even in a regular pattern, a local pattern such as an endmost pattern of it cannot be observed.
Next, as one of the pattern observation methods by use of charged particle beams, a scanning electron microscope (SEM) is known which uses electron beams as a probe. For example, in the case of observing device pattern of a semiconductor device, the SEM irradiates the device pattern with an electron beam (primary electrons), thereby producing secondary electrons from a surface of the device pattern. Then, the SEM detects the secondary electrons with a detector and generates an observation image (hereinafter referred to as secondary electron image) from detected data of the secondary electrons. The secondary electron image can be used to observe a shape of the device pattern, thereby measuring a width of the device pattern.
However, by an ordinary observation method by use of the SEM, it is difficult to clearly observe the bottom surface of patterns having a large aspect ratio by using the secondary electron image. The reasons will be described as follows. As one example, a line and space pattern (hereinafter referred to as L/S pattern) in which a line pattern and a space pattern are disposed periodically will be used as an observation target in the description.
If the space pattern has a large width as compared to a height of the line pattern, that is, the aspect ratio is small, secondary electrons generated from a bottom surface of the space pattern (hereinafter referred to as space bottom surface), when it is irradiated with an electron beam, strike side walls of the line pattern little and are emitted to an outside of the L/S pattern, thus reaching the detector. Accordingly, the space bottom surface can be observed brightly. On the other hand, if the aspect ratio is large, the secondary electrons generated from the space bottom surface readily strike the side walls of the line pattern, thus reaching the detector less. Therefore, the space bottom surface cannot brightly be observed.
Incidentally, a method is proposed for observing a space bottom surface brightly in a case where the space bottom surface is electrically continuous with a substrate kept to a ground potential and a line pattern is discontinuous with the substrate (see, for example, Japanese Patent Application Laid-Open No. 2002-270655). By this method, an entire L/S pattern serving as a sample is irradiated with an electron beam beforehand to positively charge the sample and then irradiated with the electron beam, thereby obtaining a secondary electron image. By this method, a space bottom can be observed brightly even if the aspect ratio is large. The reasons will be described as follows. If the entire L/S pattern is irradiated with an electron beam, the line pattern's top surface (hereinafter referred to as line top surface) is positively charged, but the space bottom surface is not charged because it is grounded. If the pattern is irradiated with the electron beam after such a charged state is established, the secondary electrons emitted from the space bottom surface are drawn out of the L/S pattern by the positive charge on the line top surface, thus reaching the detector. Thus, the space bottom surface can be observed brightly.